Taper gauges--or taper feeler gauges--work using the same basic principles as a typical feeler gauge: fill the gap between two surfaces using a calibrated measuring instrument to determine the distance between them. While typical feeler gauges consist of several metal blades, each with parallel contact surfaces and carefully calibrated thicknesses, taper gauges have tapered contact planes to help measure the gaps between angled surfaces. The gauges come to a narrow point at the tip of the blade, then expand out like to form a wedge.
Fan out the taper gauge blades. Choose a blade close to the nominal size of the gap being measured.
Slide the narrow, wedged side of the blade into the gap being measured. Feel how easily the blade slips in--a tight fit means the gap is smaller than the size of the blade being used, while a loosefitting blade means the gap is a larger size.
Slide the next taper gauge blade into the gap. Use either the next size down or the next size up depending on the feel of the initial blade.
Continue until a blade slides in that touches both sides of the gap without feeling tight. The measurement stamped on the blade is the size of the tapered gap.
Keep the blades of a taper gauge lubricated with oil to ensure easy sliding. Another type of taper gauge exists for measuring the width of slots. The gauge looks like a metal, wedged ruler and sometimes comes to a pointed tip. Slide this type of taper gauge into a slot, then read how the markings along the length of the tool to determine the width of the taper. The angle of the this type of taper gauge must match the angle of the taper being measured.